1. Field of the Invention
The present invention relates to an improvement in a stamper Which is used for injection-molding plastic substrates for optical disks including read-only type optical disk such as audio-disks and video-disks, direct-write-after-read (DRAW) type optical disks and erasable DRAW (E-DRAW) type optical disks which are used as document files, video files and external memories of computers.
A single drawing attached FIGS. 1(a)-1(d) illustrates conventional production steps of the stamper which is used for molding plastic substrates for optical disks.
2. Description of the Related Arts
First of all, we will refer to an attached single drawing Which illustrates typical production steps of the stamper for molding plastic substrates for optical disks.
At first, a focussed laser beam whose intensity may be modified is directed onto a thin film of photo-resist (2) coated on a polished surface of a base disk of glass (1) as shown in FIG. 1(a). The glass disk (1) is revolved so that the photo-resist is exposed in a predetermined pattern (2') of trucking grooves and/or of a series of signal pits, shown in FIG. 1(b). A master disk, FIG. 1(b), is obtained by developing the exposed photo-resist.
Then, an electro-conductive thin film is deposited by electroless plating or sputtering technique on a surface of the patterned surface of the photo-resist of the master disk thus obtained.
Finally, an uniform nickel layer having a thickness of about 300 .mu.m is deposited on the electro-conductive thin film by electro-deposition or plating process in a bath containing nickel ions. The resulting deposited nickel disk is pealed off the master disk at the electro-conductive thin film and an outer periphery of the nickel disk is trimmed to obtain a "stamper" (3), as shown in FIG. 1(c), having a predetermined diameter onto which the relief pattern of trucking grooves and/or of a series of signal pits is transferred from the master disk.
The stamper is secured on a surface of one (5) of mold halves (5, 6) in such a manner that a molding cavity (7) is defined between a front surface of the stamper and an opposite surface of another mold half (6), as shown in FIG. 1(d). When a plastic substrate is molded in the molding cavity (7) in an injection molding machine, molten resin is injected into the molding cavity (7) so that the relief pattern of trucking grooves and/or of signal pits is transferred from the stamper (3) onto a surface of the plastic substrate.
In the prior arts, it is usual practice to polish the rear surface (4) of the stamper (3) in such a manner that the surface roughness on the rear surface becomes under 0.1 S measured according to Japanese Standard JIS B 0601 in order to realize uniform thermal conduction from the nickel stamper (3) to the molded plastic disk through a contact interface between the nickel stamper surface and the plastic surface:. In fact, if the rear surface (4) of the nickel stamper is not polished, file surface roughness on the rear surface of the nickel stamper is so high that a defect so-called "orange peals" is marked on the plastic disk obtained.
Precision required in the molding of plastic substrates for optical disks is extremely higher comparing to that is required in ordinary plastic articles. In the case of the plastic substrate for optical disks, transferability of trucking grooves and/or signal pits from the stamper to the plastic disk is the most important factor in addition to the dimensional stability such as uniformity in thickness with no distortion and uniformity of optical properties such as birefringence.
In order to improve the transferability of trucking grooves and/or signal pits, molten resin must be forced onto a patterned surface of the stamper at a high pressure in the molding stage. However, under such a high pressure condition, it is difficult to release the molded plastic article from the patterned surface of the stamper. If the molded plastic disk is not release smoothly or easily from the stamper, the transferred relief pattern is spoiled during the releasing stage resulting in that a defect so-called "cloud" which is detectable even by naked eyes is left on the molded plastic disk. This means that the transferability of the relief pattern from the stamper to the molded plastic disk and the releasing property of the molded plastic disk from the stamper are in an offset relation. Therefore, it has been requested to compromise or balance the transferability and the releasing property.
A variety of stamper models have been proposed in order to improve the releasing property. For example, Japanese Patent laid-open No. 59-218,821 discloses a stamper having a surface releasing layer of organic compound coated on the relief pattern of the stamper. Japanese Patent laid-open No. 62-119,751 discloses a stamper having a surface releasing layer of oxide or nitride deposited on the relief pattern of the stamper. Japanese Patent laid-open No. 62-180,541 discloses a stamper having an oxide layer deposited on the rear surface of the stamper.
All of these solutions utilize such phenomena that slipping property of injected molten resin on the relief pattern surface of the stamper is increase or that cooling of molten resin is retarded by providing the surface coating layer made of different material other than nickel on the relief pattern surface or the rear surface of the stamper in order to balance the transferability of tracking grooves and/or signal pits and the releasability of the molded substrate from the stamper. These solutions have such a demerit that they need expensive apparatus for depositing or coating the surface layer such as a vacuum units like vacuum-deposition machine, a CVD machine or a sputtering machine, resulting in that manufacturing cost of the stamper increases. The present inventors discovered an effective solution to solve the demerit of the prior art and completed the present invention.
An object of the present invention is to provide a very simple method for retarding the cooling rate of molten resin on the stamper surface in order to improve the releasing property of the molded substrate from the stamper as well as to eliminate the defect of "cloud" which is observed in the molded substrate.